1. Field of the Invention
The present invention relates to a method for recording information on optical disks and a method for reproducing the information from the optical disks.
2. Description of Related Art
A mark edge recording method has been used for recording information in optical disk systems that use such optical disks as CD and DVD media. According to the mark edge recording method, binary data xe2x80x9c1xe2x80x9d is assigned at the boundary (edge) between a recording mark and a space (a portion between marks) and binary data xe2x80x9c0xe2x80x9d is assigned at each of other portions. In this connection, in order to realize high density (high efficiency) recording that will be made over the resolution limit of the optical spots formed on the subject optical disk, a run-length limit code (RLL code) is used to limit the number of xe2x80x9c0""sxe2x80x9d to be placed consecutively between xe2x80x9c1xe2x80x9d and xe2x80x9c1xe2x80x9d. This is why the lengths of the shortest mark and the shortest space are kept longer than the optical resolution limit while such the high density recording is made. For example, in the case of the EFM code used for the CD (Compact Disk) media and the EFMPlus (or referred to as the 8-16 code) used for the DVD media are run-length limit codes in which the minimum value of the mark/space length is limited at 3T (T: a channel clock cycle). Those codes are disclosed, for example, in the official gazette of U.S. Pat. No. 5,696,505.
Both of the EFM code and the EFMPlus code (8-16 code) are (d, k) RLL codes that satisfy d=2 and k=10. Both d=2 and k=10 mentioned here mean the minimum and maximum numbers of xe2x80x9c0""sxe2x80x9d to be placed between xe2x80x9c1xe2x80x9d and xe2x80x9c1xe2x80x9d in the run-length code. In an actual recording operation, what are recorded are not edges, but marks. Thus, xe2x80x9c1xe2x80x9d/xe2x80x9c0xe2x80x9d obtained as a result of NRZI conversion is recorded corresponding to a mark/space. In the NRZI conversion, when the run-length limit code denotes xe2x80x9c1xe2x80x9d, the code inversion is done like xe2x80x9c1xe2x80x9dxe2x86x92xe2x80x9c0xe2x80x9d and xe2x80x9c0xe2x80x9dxe2x86x92xe2x80x9c1xe2x80x9d. When the run-length limit code denotes xe2x80x9c0xe2x80x9d, the code is kept as is in the conversion. Consequently, d+1 is assumed as the minimum mark/space length. In other words, each of the minimum mark and space lengths is 3 bits in both of the EFM code and the EFMPlus code.
A typical example of the shortest run-length d=1 is the 1-7 modulation employed for optical magnetic disks. Because d=1 is assumed, the shortest mark/space length becomes 2T.
In recent years, optical disk units are also required of higher density recording performance. And, in order to meet this requirement, employment of the Partial Response Maximum Likelihood (hereinafter, to be abbreviated as PRML) comes to be under examination.
The PRML, which is an application of the conventional communication technique, has been used for magnetic disk units so as to improve the recording density. The Partial Response (PR) is a method for reproducing data while a target signal band is compressed with positive use of the mutual interference between codes (mutual interference between reproduction signals corresponding to the adjacently recorded bits). The Viterbi decoding method (ML) is a kind of so-called maximum likelihood sequence estimation method, which uses the inter-symbol interference rules of reproduction waveforms effectively to reproduce data according to the signal amplitude information obtained at plural times.
However, it has been difficult to record and reproduce data on/from any of the conventional optical disks while both high reliability and low error rate of those optical disks are satisfied.
Under such circumstances, it is an object of the present invention to provide a highly reliable optical recording/reproducing system that can always record/reproduce data at a low error rate.
The present inventor et al have analyzed and examined the above-described conventional problems. On an optical disk, the physical state usually differs between the recording mark and the space as shown in FIG. 6, so that the signal quality always comes to differ between a recording mark portion and a space portion. For example, on a phase change recording medium made on an experimental basis, the space portion becomes crystal (polycrystal) and the recording mark portion becomes amorphous when in recording data. And, the amorphous portion always has a highly uniform physical structure while the space portion is usually apt to increase noise therein due to such various factors as the variation of the polycrystal axis and the light scattering at grain boundaries. FIG. 16 shows an analysis result of the distribution of the signal fluctuation component of the above phase change recording medium. In FIG. 16, each position that is not described as a mark denotes a space. In FIG. 16, it will be found that noise components are concentrated in space portions. In this example, the noise in the space portion is about 6 dB higher than that of the mark portion. Consequently, even when microscopic marks are formed accurately, the signal quality comes to be degraded because of the signal fluctuation to occur in the spaces, thereby fast and high density recording/reproducing of data is disabled. There is another example in which recording marks are formed on a recording film made of coloring matters and an inorganic film of a write-once-read-many optical disk by making good use of the deforming of the recording film, the substrate, and the diffusion of the materials. In such a case, however, microscopic recording marks cannot be formed stably when in high density recording, so that the signal quality in such the recording marks is apt to be lower. As a result, it becomes difficult to realize fast and high density recording/reproducing of data.
In order to achieve the above object, the present invention employs a mark recording code in which the ratio between the average mark length and the average space length is changed from 1:1 so as to increase the signal components in either marks or spaces wherever is more improved in recording/reproducing signal quality.
This means that the recording code is used so that the average run-length differs between a mark portion and a space portion, because each of the mark length and the space length is fixed at run-length+one channel bit (1T). Concretely, a first state, as well as a second state that differs from the first state are formed on the recording film of the target optical disk in accordance with the RLL coding rules. And, when information is retained at the boundary between the first and second states, the average run-length of the RLL codes for recording the first state is set shorter than that of the RLL codes for recording the second state. When in phase change recording, the first state becomes crystal and the second state becomes amorphous. And, because the noise in the space portion (crystal) is larger than the noise in the mark portion (amorphous) as described above, the average space length is set shorter than the average mark length for recording information as shown in FIG. 1.
When the ratio between the average mark length and the average space length is changed from 1:1 as described above, it should be avoided to dispose a portion in which there are extremely many marks and a portion in which there are extremely less marks together in a recording track. Otherwise, the co-existence will cause a low frequency fluctuation to occur in reproduction signals and such the fluctuation will affect the servo system and/or the synchronizing system (PLL) adversely. To avoid this, it is effective to control the ratio fixedly at a value predetermined according to the medium characteristics. It is also effective for solving the problem about the average run-lengths of both mark portion and space portion.
One of the easy and simple methods for controlling the above ratio at a certain target value is controlling the DSV (Digital Sum Value) so as to get the DSV closer to a value to which a predetermined value is added at fixed time intervals when the mark portion is xe2x88x921 and the space portion is 1.
Such the coding method will be capable of suppressing the low frequency component included in the code. However, because the rate of the mark portion differs from the rate of the space portion, the DC component does not become zero; it takes a predetermined value.
On the other hand, the reproduction process is divided into a process of obtaining reproduction signals from the target recording medium with use of a pick-up head, a waveform equalizing process of correcting the frequency characteristics of the reproduction signals, a gain controlling process of setting the amplitude of each reproduction signal to a predetermined value, a process of obtaining a modulated data bit string by distinguishing each reproduction signal between xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d according to a predetermined decision level, and a process of obtaining digital information by decoding each modulated data bit string in accordance with the run-length limit code rules. The decision level is controlled appropriately so that the ratio R=N1/N0 takes a predetermined value when the number of xe2x80x9c1""sxe2x80x9d included in the modulated data bit string is assumed as N1 and the number of xe2x80x9c0""sxe2x80x9d included in the same string is assumed as N0.
FIG. 2 shows how the signal quality is improved when the ratio between mark and space is changed. In FIG. 2, a total noise level difference is calculated between when the noise in the space portion is 5 dB higher than that in the mark portion and when the noise in the space portion is 10 dB is higher than that in the mark portion. The total noise of both mark and space portions is represented by the following expression.
Ntotal=(Pmarkxc3x97(Nmark)2+Pspacexc3x97(Nspace)2)1/2xe2x80x83xe2x80x83Expression 1
where each symbol has the following meaning:
Pmark: Rate of the mark portion
Pspace: Rate of the space portion
Nmark: Noise level in the mark portion
Nspace: Noise level in the space portion
As shown in FIG. 2, for example, if the noise level in the space portion is 10 dB higher than that in the mark portion and the rate of the space portion is reduced by 20% to 0.3, then, the noise level can be reduced by about 2 dB.
As a result of the investigation of well-known examples carried out after the completion of this specification, the present inventor et al found JP-A No. 163887/2000. This official gazette describes that modulation is done for modulated data selected so that each of a bit string corresponding to a mark string and a bit string corresponding to a space string comes to have only a value quantized at a predetermined size within a range that satisfies both minimum and maximum lengths of the RLL and reproduction errors are checked according to the quantization condition at the time of demodulation. This official gazette does not describe any code that enables the average run-length to differ between the mark portion and the space portion while the present invention employs such the code.